1. Field of the Invention
The present invention relates to surface acoustic wave devices including a plurality of surface acoustic wave elements electrically connected to each other by wiring lines, and more specifically, relates to surface acoustic wave devices including three-dimensional wiring portions at which a plurality of wiring lines intersect with each other in a three-dimensional manner.
2. Description of the Related Art
To date, surface acoustic wave devices have been widely used for band-pass filters for, for example, cellular phones. In order to reduce the size of such surface acoustic wave devices, the following Japanese Unexamined Patent Application Publication No. 2006-217226 discloses a surface acoustic wave device shown in FIG. 7.
A surface acoustic wave device 101 includes a piezoelectric substrate 102 composed of, for example, LiTaO3. An IDT electrode 103 is formed on the piezoelectric substrate 102. A protective film 104 is formed so as to cover the IDT electrode 103.
Surface acoustic waves are excited by applying an AC voltage to the IDT electrode 103. A space A is formed so as not to prevent the vibration of the surface acoustic waves. That is, the space A is formed over a vibrating portion including the IDT electrode 103 so as not to prevent the vibrating portion from vibrating.
A surrounding wall 105 is formed so as to enclose the vibrating portion and thereby to form the space A. The surrounding wall 105 is formed by patterning a heat-resistant photosensitive resin. In addition, a cover 106 is stacked so as to cover the space A enclosed by the surrounding wall 105. The cover 106 is formed of a synthetic resin layer.
Moreover, through-holes extend through the surrounding wall 105 and the cover 106, and bumps 107 are disposed inside the through-holes. The bumps 107 are electrically connected to the IDT electrode 103 at predetermined portions (not shown).
The surface acoustic wave device 101 is designed such that the size, in particular, the thickness thereof is reduced by using the surrounding wall 105 composed of the photosensitive resin and the cover 106 formed of the synthetic resin layer.
When band-pass filters, for example, for cellular phones, are configured, filter circuits are often formed by electrically connecting a plurality of surface acoustic wave elements. In this case, a common piezoelectric substrate is used for the plurality of surface acoustic wave elements. That is, a plurality of vibrating portions are formed on one piezoelectric substrate so as to form the surface acoustic wave elements. Subsequently, the plurality of vibrating portions are electrically connected to each other by wiring lines formed on the piezoelectric substrate. In this case, the plurality of wiring lines intersect with each other in a three-dimensional manner at some portions so that the size of the device is reduced. As shown in FIG. 8, for example, a first wiring line 112 and a second wiring line 113 intersect with each other in a three-dimensional manner at a three-dimensional wiring portion 111. An insulating layer 114 is formed between the first wiring line 112 and the second wiring line 113.
However, when the piezoelectric substrate is commonly used for a plurality of surface acoustic wave elements having a structure with which the size and the thickness of the devices can be reduced as in the surface acoustic wave device 101 described in Japanese Unexamined Patent Application Publication No. 2006-217226, ruptures disadvantageously occur in wiring lines at the three-dimensional wiring portions. For example, when two or more surface acoustic wave elements 101 and at least one three-dimensional wiring portion 111 shown in FIG. 8 are formed on one piezoelectric substrate, the three-dimensional wiring portion is disposed between the surrounding wall 105 and the piezoelectric substrate 102. Therefore, the second wiring line 113, located at an upper position, is disposed between the insulating layer 114 and the surrounding wall 105.
The insulating layer 114 is formed so as to improve characteristics of the device by reducing the parasitic capacitance between the first wiring line 112 and the second wiring line 113, and to ensure the electrical insulation at the three-dimensional intersecting portion. To this end, the insulating layer 114 is composed of a synthetic resin such as polyimide, which differs from the synthetic resin of the surrounding wall 105.
Therefore, when the ambient temperature changes, ruptures sometimes occur at the second wiring line 113 due to stress caused by a difference in the coefficients of linear expansion between the surrounding wall 105 disposed over the second wiring line 113 and the insulating layer 114 disposed under the second wiring line.